As creatives, we have the option of licensing rights to our work, or of selling our writing outright. Much of my work -- my business writing and copywriting -- is work done for hire. As soon as my invoice is paid, the buyer gets all rights to the work.
When you sell all rights to something, that work has gone for good. You can't reuse it, or resell it. Therefore it's important that if writing (or any other creative occupation) is your fulltime work, you devote some of your working time to creating products which you can license.
For writers, these products could include books (fiction and nonfiction), magazine articles, scripts, and ebooks.
This is building your inventory, which I covered in this article.
Be aware of rights issues, and of which rights you're selling, at all times. When a magazine editor offers you fifty cents a word for FNASR (First North American Serial Rights) you need to know exactly what that means. It means that you can still sell second NASR, and you've got the rest-of-world rights to play with too. I'm in Australia, so for short magazine articles, I'm quite happy to sell First Australian Serial Rights quite cheaply, because I know I've got lots of rights still to sell -- although "license" is a better term, because when you "sell" rights, you're licensing your work for a specific use and for a set period.
If you're not a hundred per cent sure of how copyright and the rights to your work operate, please buy a book on the subject. It's worth spending the money, to have the information at your fingertips.
When you know how rights work, you can ask an editor who's offered you a dollar a word what rights she's buying. If (horrors) she tells you she wants all rights to the piece for a dollar a word, that perceived good price starts to look shabby if you've been intending to use the material in other ways: as a chapter in a book, for example, or if you've been counting on selling only FNASR, and wanted to sell UK rights as well.